1. Field of the Invention
The present invention relates to display devices.
2. Description of the Prior Art
With rapid development of planar displays, more and more planar display technologies are being researched for increasing product competitiveness. In order to meet the needs of demanding applications, the flat panel industry is now looking at displays known as active-matrix organic light emitting displays (AMOLEDs). An AMOLED has an integrated electronic back plane as its substrate and is particularly suitable for high-resolution, high-information content applications including videos and graphics. This form of display is made possible by the development of polysilicon technology, which, because of its high carrier mobility, provides thin-film-transistors (TFTs) with high current carrying capability and high switching speed. In an AMOLED display, each individual pixel can be addressed independently via the associated driving thin-film transistors (TFTs) and capacitors in the electronic back plane.
FIG. 1 shows a configuration of a prior art AMOLED 10. The AMOLED 10 includes a plurality of pixels 100 arranged in a matrix manner, and only one pixel is shown in FIG. 1 for simplicity. The pixels 100, each including an organic light emitting diode (OLED) 102 as a pixel light emitting device, are coupled to voltage sources VDD and VEE, and to external driving circuits via corresponding gate lines 12 and data lines 14. Each pixel 100 further includes a storage capacitor 104, an n-type control TFT 106, and a p-type driving TFT 108. In each pixel 100, a gate and a drain of the control TFT 106 is coupled to the gate line 12 and the data line 14, respectively, while a gate and a source of the driving TFT 108 is coupled to a source of the control TFT 106 and the voltage source VDD, respectively. The storage capacitor 104 is coupled between the gate and the source of the driving TFT 108. The OLED 102 is coupled between a drain of the driving TFT 108 and the voltage source VEE.
An operation of the AMOLED 10 will be described. First, a gate signal is generated by an external gate driving circuit and sent to the gate line 12 for switching on the control TFT 106. Then, a signal voltage that has been supplied from an external data driving circuit to the data line 14 is input to the gate of the driving TFT 108 and to the storage capacitor 104 via the turned-on control TFT 106. The driving TFT 108 supplies a driving current according to the signal voltage to the OLED 102, causing it to illuminate in response to the signal voltage.
As well-known to those skilled in the art, a TFT has three working modes: cut-off, linear, and saturation. For example, the drain current of an n-type TFT can be represented by the following formulae:Id_off=0, when Vgs<Vth  (1)Id_linear=μCOXWeffLeff [(Vgs−Vth)Vds−Vds2/2], when 0<Vds<Vgs−Vth  (2)Id_sat=[μCOXWeffLeff (Vgs−Vth)2]/2, when 0<Vgs−Vth<Vds  (3)                where μ is the effective surface mobility of the carriers;                    COX is the gate oxide capacitance;            Weff is the effective channel width;            Leff is the effective channel length;            Vgs is the voltage established between the gate and the source of the TFT;            Vds is the voltage established between the drain and the source of the TFT;            Vth is the threshold voltage of the TFT;            Id_off is the drain current when the TFT works in the cut-off mode;            Id_linear is the drain current when the TFT works in the linear region;            Id_sat is the drain current when the TFT works in the saturation region.                        
Regardless of doping types, when a transistor begins to conduct depends on its threshold voltage Vth, which is characterized by the gate conductor/insulator material, the thickness of gate oxide material and the channel doping concentration. The threshold voltage Vth of a TFT can deviate from its typical voltage setting for various reasons, such as due to process variations or changes of operational environment. FIG. 2 shows a current-voltage (I-V) curve of the driving TFT 108 and the OLED 102. In FIG. 2, a curve A represents the I-V curve of the OLED 102, a curve B represents the I-V curve of the driving TFT 108 with a nominal threshold voltage Vth, and curves B′ and B″ represent the I-V curves of the driving TFT 108 when the threshold voltage deviates from the nominal value Vth to Vth′ and Vth″, respectively. As shown in FIG. 2, the designed operational point S (indicated by “” in FIG. 2) of the OLED 102 can shift to points S′ and S″ (indicated by “X” in FIG. 2) with threshold voltage deviations. As represented by the formula (1), the luminance of the OLED 102 depends largely on the threshold voltage Vth of the driving TFT 108, whose I-V characteristic is a function of the threshold voltage Vth raised to the second power when working in the saturation region. The pixels 100 can have irregular display uniformity (mura) when displaying images of the same gray scale if the threshold voltages Vth of the corresponding driving TFTs 108 deviate from the nominal value. Therefore, the prior art AMOLED 10 has poor display uniformity even with slight variation of TFT characteristics.